Tailoring Energy Transfer in Mixed Eu/Tb Metal–Organic Frameworks for Ratiometric Temperature Sensing

Eu/Tb metal–organic frameworks (Eu/Tb-MOFs), exhibiting Eu3+ and Tb3+ emissions, stand out as some of the most fascinating luminescent thermometers. As the relative thermal sensitivity model is limited to its lack of precision for fitting ratio of Eu3+ and Tb3+ emissions, accurately predicting the sensing performance of Eu/Tb-MOFs remains a significant challenge. Herein, we report a series of luminescent Eu/Tb-MOF thermometers, EuxTb1−xL, with excellent thermal sensitivity around physiological levels, achieved through the tuning energy transfer from ligands to Eu3+ and Tb3+ and between the Ln ions. It was found that the singlet lowest-energy excited state (S1) of the ligand and the higher triplet energy level (Tn) are crucial in the energy transfer processes of ligand→Tb3+ and ligand→Eu3+. This enables EuxTb1−xL to serve as an effective platform for exploring the impact of these energy transfer processes on the temperature-sensing properties of luminescent Eu/Tb-MOF thermometers. The relative thermal sensitivity is comparable to that of dual-center MOF-based luminescent thermometers operating at physiological levels. This study provides valuable insights into the design of new Eu/Tb thermometers and the accurate prediction of their sensing performance.

Given that excited-state calculations of Eu/Tb-MOFs are time-consuming, the Mott-Seitz model is frequently employed as a rapid experimental approach to rationalize the thermal quenching of luminescence in these materials [32][33][34].However, a significant limitation of the Mott-Seitz model is its lack of precision when used to fit the ratio between the integrated intensities of Tb 3+ and Eu 3+ emissions in Eu/Tb materials.This shortcoming is often attributed to the empirical observation [35][36][37][38][39][40] that Tb 3+ emission is influenced by the energy difference (∆E T ) between the triplet lowest-energy excited state (T 1 ) of the ligand and the lowest emitting levels of Tb 3+ .Conversely, the model assumes that the Eu 3+ transition is unaffected by temperature variations.Consequently, the integrated intensity ratio is predominantly determined by ∆E T .However, this prediction is not consistently observed in practical applications.In addition to T 1 , higher energy levels of ligands, such as the singlet lowest-energy excited state (S 1 ), may also play a crucial role in the energy transfer processes between the ligand and Tb 3+ /Eu 3+ in certain Eu/Tb-MOFs.Moreover, recent findings indicate that Eu 3+ content is another significant structural factor.Therefore, considerable efforts are still needed to explore the contributions of energy transfers between the higher energy levels of ligands and Tb 3+ /Eu 3+ , as well as the impact of Eu 3+ content on the temperature-sensing performance of Eu/Tb-MOF thermometers.
Herein, we report the investigation of TbL and Eu x Tb 1−x L (x = 0.0001, 0.0005, and 0.001).Aiming to bridge the gap between the composition of Eu/Tb-MOFs and their desired thermometric performance, we examined the temperature-dependent luminescence of TbL and Eu x Tb 1−x L. This study demonstrates that energy transfer likely occurs from S 1 and higher triplet energy levels (T n ) of the ligand to Tb 3+ or Eu 3+ and establishes how to optimize the thermometric performance of Eu x Tb 1−x L by selecting appropriate ligands and adjusting Eu 3+ content.The results offer valuable insights for further improvements in the rational design of Eu/Tb-MOF luminescent thermometers, enabling their effective operation across temperature ranges from cryogenic to physiological levels.

Structural Properties of TbL and Eu x Tb 1−x L
In recent years, we have reported the structural and luminescence properties of a Tb-MOF and a series of Eu/Tb-MOFs based on a tetracarboxylic acid ([1,1:4,1-terphenyl]-2,4,4,5-tetracarboxylic acid, H 4 L) [41], referred to as TbL and Eu x Tb 1−x L, respectively.Despite the triplet excited-state energy of H 4 L (20,661 cm −1 ) being very close to the 5 D 4 energy level of Tb 3+ (20,500 cm −1 ), TbL emits bright green light originating from Tb 3+ upon UV excitation.For Eu x Tb 1−x L, both the 5 D 4 → 7 F 5 transition of Tb 3+ and the 5 D 0 → 7 F 2 transition of Eu 3+ are observed in the emission spectra.These results make Eu x Tb 1−x L an excellent platform to investigate the effects of S 1 , T n , and Eu 3+ content on temperaturesensing properties.
TbL and Eu x Tb 1−x L are in soluble in water and common organic solvents such as methanol and ethanol.Figure 1 shows the coordination environment of Tb 3+ ions, L 4− ligands, and supramolecular interactions within the crystal structure of TbL.The asymmetric unit of TbL includes two crystallographically unique Tb 3+ ions (Tb1 and Tb2), two L 4− ligands (one of which is on the inversion center), and two coordinated NMP molecules.As shown in Figure 1a,b, there exist µ 7 and µ 8 coordination modes for the ligand.Two Tb1 and two Tb2 are linked by 12 carboxylates and 4 NMP to form the Tb 4 (COO) 12 •4NMP cluster (Figure 1c).Importantly, a C-H•••π hydrogen bond with a 2.456 Å H•••Cg length (C38-H38A→Cg8) was observed between the ligand on the inversion center and the NMP molecule coordinated with Tb1 (Figure 1d).In contrast, π-π stacking was not observed.This result indicates that Tb 3+ emission is sensitized by excited ligand monomers.The structures of TbL and Eu x Tb 1−x L were confirmed by powder X-ray diffraction (PXRD) (Figure S1), thermogravimetric analysis (TGA) (Figure S2), and Fourier transform infrared (FT-IR) spectroscopy (Figure S3).For TbL and Eu x Tb 1−x L, TGA curves were very similar in the temperature range from room temperature to 723 K.At higher temperatures, the TGA curves of two compounds with x = 0.0001 and 0.0005 were different from the other ones.This is probably due to complex phase changes in combustion of the ligand molecules in the temperature range.The characteristic IR absorption bands of amide C=O vibration and sp 3 C−H vibration in NMP are observed around 1670 and 2890 cm −1 , respectively.The IR spectra difference, near 3500 cm −1 , between the compounds resulted from adsorbed water on the surface of particles.These results demonstrate that the samples of TbL and Eu x Tb 1−x L (x = 0.0001, 0.0005, and 0.001) are essentially pure phases.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 11 curves of two compounds with x = 0.0001 and 0.0005 were different from the other ones.This is probably due to complex phase changes in combustion of the ligand molecules in the temperature range.The characteristic IR absorption bands of amide C=O vibration and sp 3 C−H vibration in NMP are observed around 1670 and 2890 cm −1 , respectively.The IR spectra difference, near 3500 cm −1 , between the compounds resulted from adsorbed water on the surface of particles.These results demonstrate that the samples of TbL and EuxTb1−xL (x = 0.0001, 0.0005, and 0.001) are essentially pure phases.

Photoluminescence Properties of TbL and EuxTb1−xL
As reported in prior research, the ΔET of TbL is only 161 cm −1 , which contradicts the empirical rule.To investigate the excited states of the ligand, we examined the luminescence properties of the ligand.Concentration-dependent 3D photoluminescence (PL) spectra of H4L were recorded at room temperature and identified as ligand monomer emissions.As depicted in Figure 2a-d, the maximum excitation wavelengths of the H4L ligand in solutions shift from 318 nm to 353 nm as the concentrations increase from 10 −4 to 10⁻ 2 M, while the emission peaks remain at 387 nm.The S1 state of H4L molecules is thus determined to be 27,400 cm −1 .For solid H4L, the emission maximum redshifts to 442 nm and the excitation maximum to 392 nm, likely due to molecular packing effects.
Next, phosphorescence spectra of the ligand were recorded at 77 K in the solid state upon excitation from 260 nm to 365 nm (Figure 2e,f).The band at 526 nm was assigned to T1, and the weak band around 420 nm, ~23,800 cm −1 , was attributed to the ligand's Tn state.The energy difference between the Tn state and the emitting level 5 D4 is approximately 3300 cm −1 , suggesting that the Tn→ 5 D4 energy transfer might also contribute to the observed bright TbL emission.Therefore, we speculate that TbL emission may result from S1 and higher triplet energy levels of the ligand.A recent example of S1→Ln 3+ energy transfer was reported by Jérôme Long and Luís D. Carlos et al., in which the intramolecular energy transfer rates of [Ln(bpy)2(NO3)3] (bpy-2,2′-bipyridine; Ln-Tb or Eu) [26] were determined.At temperatures above 125 K, the S1→Eu 3+ channel dominates the sensitization of the 5 D0 level.

Photoluminescence Properties of TbL and Eu x Tb 1−x L
As reported in prior research, the ∆E T of TbL is only 161 cm −1 , which contradicts the empirical rule.To investigate the excited states of the ligand, we examined the luminescence properties of the ligand.Concentration-dependent 3D photoluminescence (PL) spectra of H 4 L were recorded at room temperature and identified as ligand monomer emissions.As depicted in Figure 2a-d, the maximum excitation wavelengths of the H 4 L ligand in solutions shift from 318 nm to 353 nm as the concentrations increase from 10 −4 to 10 −2 M, while the emission peaks remain at 387 nm.The S 1 state of H 4 L molecules is thus determined to be 27,400 cm −1 .For solid H 4 L, the emission maximum redshifts to 442 nm and the excitation maximum to 392 nm, likely due to molecular packing effects.
Next, phosphorescence spectra of the ligand were recorded at 77 K in the solid state upon excitation from 260 nm to 365 nm (Figure 2e,f).The band at 526 nm was assigned to T 1 , and the weak band around 420 nm, ~23,800 cm −1 , was attributed to the ligand's T n state.The energy difference between the T n state and the emitting level 5 D 4 is approximately 3300 cm −1 , suggesting that the T n → 5 D 4 energy transfer might also contribute to the observed bright TbL emission.Therefore, we speculate that TbL emission may result from S 1 and higher triplet energy levels of the ligand.A recent example of S 1 →Ln 3+ energy transfer was reported by Jérôme Long and Luís D. Carlos et al., in which the intramolecular energy transfer rates of [Ln(bpy) 2 (NO 3 ) 3 ] (bpy-2,2 ′ -bipyridine; Ln-Tb or Eu) [26] were determined.At temperatures above 125 K, the S 1 →Eu 3+ channel dominates the sensitization of the 5 D 0 level.

Temperature-Dependent Photoluminescence Properties of TbL and EuxTb1−xL
To rationalize the energy diagram of this system, we consider the energy transfer from the ligand to Tb 3+ and the backward energy transfer from Tb 3+ to the ligand in relation to the temperature-dependent 5 D4 emissions and lifetime.Upon excitation at 351 nm, TbL shows typical emissions around 488, 542, 583, and 621 nm, corresponding to the 5 D4→ 7 F6-3 transitions (Figure 3a).The relative intensities of the 5 D4→ 7 F5 (ITb) transitions were quantified by integrating the emission spectra between 530 and 570 nm.The ITb exhibited negative thermal quenching in the range of 77-225 K, with a 63% increase, and mild thermal quenching in the range of 22-353 K, with an ≈42% decrease (Figure 3b).The temperature sensitivity of TbL is much lower than that of (Me2NH2)3[Ln3(FDC)4(NO3)4]•4H2O (H2FDC = 9-fluorenone-2,7-dicarboxylic acid) [36], with a prior sample showing low ΔET, suggesting a low backward energy transfer rate for TbL.As the first ΔET-driven single-lanthanide organic framework ratiometric luminescent thermometer, (Me2NH2)3[Ln3(FDC)4(NO3)4]•4H2O showed extreme Eu 3+ emission thermal quenching with increasing temperature.This result aligns with the small energy difference (553 cm −1 ) between the H2FDC triplet excited state (17,794 cm −1 ) and the 5 D0 Eu 3+ level (17,241 cm −1 ), indicating a strong thermally activated ion-to-ligand backward energy transfer.We obtained the activation energies for the nonradiative channels of (Me2NH2)3[Ln3(FDC)4(NO3)4]•4H2O and found that the values match the experimentally observed energy difference, proving that ion-to-ligand backward energy transfer is the dominant pathway for Eu 3+ emission thermal quenching.These results suggest that the empirical observations overestimate the contribution of T1 level to energy transfer between ligands and Ln 3+ in certain Ln-MOFs.
Figure 3c shows the temperature-dependent 5 D4 decay curves of TbL, which can only be accurately represented by biexponential decay functions, possibly due to the presence of two distinct Tb 3+ local sites.The determined lifetimes range from 1118 to 198 µs between 77 and 353 K.As it is beyond the scope of the present paper to solve the energy transfer rate and backward energy transfer rate between the S1, Tn, T1, and 5 D4 levels, we adopted the empirical Mott-Seitz model here.To rationalize the energy diagram of this system, we consider the energy transfer from the ligand to Tb 3+ and the backward energy transfer from Tb 3+ to the ligand in relation to the temperature-dependent 5 D 4 emissions and lifetime.Upon excitation at 351 nm, TbL shows typical emissions around 488, 542, 583, and 621 nm, corresponding to the 5 D 4 → 7 F 6-3 transitions (Figure 3a).The relative intensities of the 5 D 4 → 7 F 5 (I Tb ) transitions were quantified by integrating the emission spectra between 530 and 570 nm.The I Tb exhibited negative thermal quenching in the range of 77-225 K, with a 63% increase, and mild thermal quenching in the range of 22-353 K, with an ≈42% decrease (Figure 3b).The temperature sensitivity of TbL is much lower than that of (Me 2 NH 2 ) 3 [Ln 3 (FDC) 4 (NO 3 ) 4 ]•4H 2 O (H 2 FDC = 9-fluorenone-2,7-dicarboxylic acid) [36], with a prior sample showing low ∆E T , suggesting a low backward energy transfer rate for TbL.As the first ∆E T -driven single-lanthanide organic framework ratiometric luminescent thermometer, (Me 2 NH 2 ) 3 [Ln 3 (FDC) 4 (NO 3 ) 4 ]•4H 2 O showed extreme Eu 3+ emission thermal quenching with increasing temperature.This result aligns with the small energy difference (553 cm −1 ) between the H 2 FDC triplet excited state (17,794 cm −1 ) and the 5 D 0 Eu 3+ level (17,241 cm −1 ), indicating a strong thermally activated ion-to-ligand backward energy transfer.We obtained the activation energies for the nonradiative channels of (Me 2 NH 2 ) 3 [Ln 3 (FDC) 4 (NO 3 ) 4 ]•4H 2 O and found that the values match the experimentally observed energy difference, proving that ion-to-ligand backward energy transfer is the dominant pathway for Eu 3+ emission thermal quenching.These results suggest that the empirical observations overestimate the contribution of T 1 level to energy transfer between ligands and Ln 3+ in certain Ln-MOFs.
1115 µs, 36.8, and ≈500-600 cm , respectively.Comparing the energy barrier value extracted from the phosphorescence spectrum (161 cm −1 ) with that resulting from the Mott-Seitz analysis, we observed a factor of ≈3 between them.This discrepancy might have arisen because of (i) the presence of forward and backward energy transfer between the S1 or higher triplet energy levels of the ligand and Tb 3+ in TbL and (ii) the fact that the Mott-Seitz model generally overestimates the energy barrier value.According to the results, it is reasonable to assume that the S1 and Tn are involved in the luminescence of EuxTb1−xL as much as the T1 state is.Thus, exploring EuxTb1−xL could provide a good platform to construct luminescent thermometers with tunable relative thermal sensitivity and operating temperature ranges and to study the contributions of multiple energy transfers and Eu 3+ content to the temperature-sensing performance of Eu/Tb-MOF thermometers.The room temperature photoluminescence (PL) excitation spectra of EuxTb1−xL were monitored at the 5 D4→ 7 F5 (Tb 3+ ) transition (Figure S3a, Supporting Information), and all excitation spectra were dominated by a broad band around 340 nm, attributed to the ligand's singlet excited state.The emission spectra of EuxTb1−xL (77-353 K) consisted of characteristic 5 D4→ 7 F6-3 transitions of Tb 3+ and 5 D0→ 7 F0-4 transitions of Eu 3+ (see Figure S3b).The relative intensities of the 5 D4→ 7 F5 (ITb) and 5 D0→ 7 F2 (IEu) transitions were quantified by integrating the emission spectra between 530-570 nm and 605-640 nm, respectively.To determine absolute temperature, ITb serves as the temperature probe while IEu acts as the reference due to their strong PL intensities, thus enabling precise temperature determination.
For the EuxTb1−xL samples, while the Tb 3+ emissions followed typical Mott-Seitz model curves across the entire temperature range, the Eu 3+ emissions displayed very distinct temperature dependence.For the sample with x = 0.0001 (Figure 4a,b), the Tb 3+ and Figure 3c shows the temperature-dependent 5 D 4 decay curves of TbL, which can only be accurately represented by biexponential decay functions, possibly due to the presence of two distinct Tb 3+ local sites.The determined lifetimes range from 1118 to 198 µs between 77 and 353 K.As it is beyond the scope of the present paper to solve the energy transfer rate and backward energy transfer rate between the S 1 , T n , T 1 , and 5 D 4 levels, we adopted the empirical Mott-Seitz model here.
We used the empirical Mott-Seitz model to fit the temperature-dependent 5 D 4 lifetimes (Figure 3d) in the 77-353 K range with R 2 = 0.98.Here, τ 0 represents the lifetimes of the Tb 3+ local sites at T = 0 K, α is the ratio between the nonradiative (T = 0 K) and radiative rates, ∆E represents the activation energy for the nonradiative channels of Tb 3+ , k B is the Boltzmann constant, and T is the absolute temperature.τ 0 , α, and ∆E are determined to be 1115 µs, 36.8, and ≈500-600 cm −1 , respectively.Comparing the energy barrier value extracted from the phosphorescence spectrum (161 cm −1 ) with that resulting from the Mott-Seitz analysis, we observed a factor of ≈3 between them.This discrepancy might have arisen because of (i) the presence of forward and backward energy transfer between the S 1 or higher triplet energy levels of the ligand and Tb 3+ in TbL and (ii) the fact that the Mott-Seitz model generally overestimates the energy barrier value.
According to the results, it is reasonable to assume that the S 1 and T n are involved in the luminescence of Eu x Tb 1−x L as much as the T 1 state is.Thus, exploring Eu x Tb 1−x L could provide a good platform to construct luminescent thermometers with tunable relative thermal sensitivity and operating temperature ranges and to study the contributions of multiple energy transfers and Eu 3+ content to the temperature-sensing performance of Eu/Tb-MOF thermometers.The room temperature photoluminescence (PL) excitation spectra of Eu x Tb 1−x L were monitored at the 5 D 4 → 7 F 5 (Tb 3+ ) transition (Figure S3a, Supporting Information), and all excitation spectra were dominated by a broad band around 340 nm, attributed to the ligand's singlet excited state.The emission spectra of Eu x Tb 1−x L (77-353 K) consisted of characteristic 5 D 4 → 7 F 6-3 transitions of Tb 3+ and 5 D 0 → 7 F 0−4 transitions of Eu 3+ (see Figure S3b).The relative intensities of the 5 D 4 → 7 F 5 (I Tb ) and 5 D 0 → 7 F 2 (I Eu ) transitions were quantified by integrating the emission spectra between 530-570 nm and 605-640 nm, respectively.To determine absolute temperature, I Tb serves as the temperature probe while I Eu acts as the reference due to their strong PL intensities, thus enabling precise temperature determination.
For the Eu x Tb 1−x L samples, while the Tb 3+ emissions followed typical Mott-Seitz model curves across the entire temperature range, the Eu 3+ emissions displayed very distinct temperature dependence.For the sample with x = 0.0001 (Figure 4a,b), the Tb 3+ and Eu 3+ emissions remained relatively constant in the range of 77-175 K, followed by decreases of approximately 92% and 75% up to 353 K, respectively.For the sample with higher Eu 3+ content (x = 0.0005), the Tb 3+ emissions underwent typical thermal quenching, decreasing by approximately 88% up to 353 K.Meanwhile, the Eu 3+ emissions displayed negative thermal quenching in the range of 77-253 K (Figure 4c,d), with an increase of approximately 26% followed by a decrease of approximately 23% up to 353 K.With increasing Eu 3+ content (x = 0.001), the Eu 3+ emissions showed stronger negative thermal quenching, rising by ≈270% up to 353 K (Figure 4e,f), while the Tb 3+ emissions decreased by ≈90% up to 353 K.This indicates that the thermal sensitivity of Eu 3+ emissions progressively increases with rising Eu 3+ content, making Eu x Tb 1−x L excellent candidates for ratiometric thermometers with tunable thermometric performances.
Molecules 2024, 29, x FOR PEER REVIEW 6 of 1 Eu 3+ emissions remained relatively constant in the range of 77-175 K, followed by de creases of approximately 92% and 75% up to 353 K, respectively.For the sample with higher Eu 3+ content (x = 0.0005), the Tb 3+ emissions underwent typical thermal quenching decreasing by approximately 88% up to 353 K.Meanwhile, the Eu 3+ emissions displayed negative thermal quenching in the range of 77-253 K (Figure 4c,d), with an increase o approximately 26% followed by a decrease of approximately 23% up to 353 K.With in creasing Eu 3+ content (x = 0.001), the Eu 3+ emissions showed stronger negative therma quenching, rising by ≈270% up to 353 K (Figure 4e,f), while the Tb 3+ emissions decreased by ≈90% up to 353 K.This indicates that the thermal sensitivity of Eu 3+ emissions progres sively increases with rising Eu 3+ content, making EuxTb1−xL excellent candidates for rati ometric thermometers with tunable thermometric performances.Here, the ratio of 5 D4→ 7 F5 transition to IEu was used to define the thermometric pa rameter Δ = ITb/IEu.We note that while ITb corresponds only to the 5 D4→ 7 F5 transition, th IEu integration range includes a small contribution from the Tb 3+ 5 D0→ 7 F3 transition.Nev ertheless, all the thermometric parameters of the EuxTb1−xL samples were fitted to an em Here, the ratio of 5 D 4 → 7 F 5 transition to I Eu was used to define the thermometric parameter ∆ = I Tb /I Eu .We note that while I Tb corresponds only to the 5 D 4 → 7 F 5 transition, the I Eu integration range includes a small contribution from the Tb 3+ 5 D 0 → 7 F 3 transition.Nevertheless, all the thermometric parameters of the Eu x Tb 1−x L samples were fitted to an empirical sigmoidal Boltzmann function, where I(Tb) is the emission intensity in the wavelength range of 530-570 nm, I(Eu) is the emission intensity in the wavelength range of 605-640 nm, A 1 represents the maximum emission intensity and A 2 represents the minimum emission intensity, T 0 is the temperature at which the emission intensity reaches half of A 1 , and T is the absolute temperature.The fit results show R 2 > 0.999, implying that the thermometer performance was unaffected by the presence of the 5 D 0 → 7 F 3 transition.This holds true for all the Eu 3+ contents studied in this study.Therefore, in the following analysis, we considered the commonly assumed labeling of I Eu as being solely due to the 5 D 0 → 7 F 2 contribution.Figure 5a displays the calibration curves of Eu x Tb 1−x L samples within the temperature range of 77-353 K under 335 nm excitation.The fitting parameters for the Eu x Tb 1−x L compounds are listed in Table 1 and show a nearly constant ∆ 0 and an increase in T 0 as the Eu 3+ content increased.By increasing the amount of Eu 3+ from x = 0.0001 to 0.001, a shift of 95 K toward lower temperatures was observed in T 0 .
lower temperatures was observed in T0.The relative sensitivity was employed to compare the performance of this model with those of similar thermometers reported in previous studies, defined as The maximum relative temperature sensitivities of the thermometers were 0.86% K −1 at 333 K, 1.42% K −1 at 293 K, and 2.11%K −1 at 273 K for, respectively, the samples with x from 0.0001 to 0.001 (Figure 5b).Intriguingly, in the physiological range, the relative sensitivities were still above 1.3%K −1 for the sample with x = 0.0005 and 1.5% K −1 for the sample with x = 0.001, while the corresponding temperature uncertainties were estimated from yielding the values of 0.08 K for the sample with x = 0.0005 and 0.07 K for the sample with x = 0.001 at 313 K (Figure 5c).This variation in the Sr is partially explained by the increase in the nonradiative decay rate of the Tb 3+ 5 D4→ 7 F5 transition (relatively to the radiative one) with the increase in Eu 3+ doping.The relative sensitivity was employed to compare the performance of this model with those of similar thermometers reported in previous studies, defined as The maximum relative temperature sensitivities of the thermometers were 0.86% K −1 at 333 K, 1.42% K −1 at 293 K, and 2.11% K −1 at 273 K for, respectively, the samples with x from 0.0001 to 0.001 (Figure 5b).Intriguingly, in the physiological range, the relative sensitivities were still above 1.3%K −1 for the sample with x = 0.0005 and 1.5% K −1 for the sample with x = 0.001, while the corresponding temperature uncertainties were estimated from yielding the values of 0.08 K for the sample with x = 0.0005 and 0.07 K for the sample with x = 0.001 at 313 K (Figure 5c).This variation in the S r is partially explained by the increase in the nonradiative decay rate of the Tb 3+ 5 D 4 → 7 F 5 transition (relatively to the radiative one) with the increase in Eu 3+ doping.

Energy Transfer
To rationalize the thermal dependence of the luminescence of the materials, we further analyzed the energy transfer from Tb 3+ ions to Eu 3+ ions using the sample with x = 0.001 as an example.Generally, the long distances between these ions may result in low rates that cannot compete with the ligand-to-Ln 3+ rates, which are orders of magnitude higher.The thermal dependence of the Tb 3+ and Eu 3+ lifetimes was investigated by monitoring the emission decay curves of the 5 D 4 → 7 F 5 and 5 D 0 → 7 F 2 transitions (Figure 5d,e), respectively.All the decay curves are well modeled and show that the 5 D 0 lifetime remains stable up to 1289 µs at 175 K, slightly decreasing to 1104 µs at 353 K.As the temperature rose from 77 K to 353 K, the 5 D 4 lifetime progressively decreased from 1239 µs to 127 µs, remaining nearly independent of the Eu 3+ doping until T > 175 K (Figure 5f).Notably, a rise time, dependent on the temperature, occurred in the 5 D 0 emission decay curves in the temperature range from 175 K to 353 K (Figure 5e).This rise time was also observed in [Ln(bpy) 2 (NO 3 ) 3 ] by Jérôme Long and Luís D. Carlos et al. [26] and was found to be similar to the 5 D 4 lifetime.This implies that the 5 D 4 level could support Tb 3+ -Eu 3+ energy transfer within Eu 0.001 Tb 0.999 L above 175 K.

Materials and Characterization
All chemicals used in this work were commercially available and used without further purification.The X-ray powder diffraction (PXRD) patterns were collected using a D/MAX 2500/PC powder diffractometer (Rigaku, Tokyo, Japan) equipped with a Cu Kα radiation source, covering a 2θ range of 5-50 • .The FT-IR spectra of samples embedded in KBr pellets were recorded using a PerkinElmer FT-IR spectrometer.Thermogravimetric analysis (TGA) was conducted on a TG/DTA 6300 thermal gravimetric analyzer (Hitachi, Tokyo, Japan) at a constant rate of 10 K/min.Room temperature luminescence spectra were obtained using an FS5 steady-state transient fluorescence spectrometer with a 150 W CW ozone-free xenon lamp.Temperature-dependent luminescence spectra were collected using an FLS1000 photoluminescence spectrometer (Edinburgh Instruments Ltd., Livingston, UK) with a 300 W CW ozone-free xenon lamp.

Preparation of [Tb 2 L 1.5 (NMP) 2 ] n (TbL)
A solution of H 4 L was prepared by dissolving H 4 L (0.2 mmol, 81.2 mg) in 24 mL of N-methyl-2-pyrrolidone (NMP), and a Tb(NO 3 ) 3 solution was prepared by dissolving Tb(NO 3 ) 3 •6H 2 O (0.4 mmol, 180.5 mg) in 24 mL of H 2 O.These two solutions were then mixed in a 100 mL Teflon-lined stainless-steel autoclave, followed by the addition of

Figure 2 .
Figure 2. Three-dimensional PL spectra of (a) 10 −4 M, (b) 10 −3 M, and (c) 10 −2 M H 4 L solutions and (d) H 4 L in solid state.Phosphorescence spectra of H 4 L (e) in solid state at 77 K and (f) in range of 380-460 nm.

2. 3 .
Temperature-Dependent Photoluminescence Properties of TbL and Eu x Tb 1−x L

Figure 4 .
Figure 4. (a) Temperature-dependent PL spectra and (b) integrated areas of I Tb and I Eu of Eu x Tb 1−x L (x = 0.0001) upon excitation at 345 nm.(c) Temperature-dependent PL spectra and (d) integrated areas of I Tb and I Eu of Eu x Tb 1−x L (x = 0.0005) upon excitation at 345 nm.(e) Temperature-dependent PL spectra and (f) integrated areas of I Tb and I Eu of Eu x Tb 1−x L (x = 0.001) upon excitation at 345 nm.Asterisk represents the overlap between Eu 3+ 5 D 0 → 7 F 2 and Tb 3+ 5 D 4 → 7 F 3 transitions, and pound sign represents that between Eu 3+ 5 D 0 → 7 F 0,1 and Tb 3+ 5 D 4 → 7 F 4 transitions.

Figure 5 .Figure 5 .
Figure 5. (a) Thermometric parameters, (b) relative thermal sensitivities, and (c) temperature uncertainties of EuxTb1−xL samples within the temperature range of 77-353 K. Temperature-dependent PL Figure 5. (a) Thermometric parameters, (b) relative thermal sensitivities, and (c) temperature uncertainties of Eu x Tb 1−x L samples within the temperature range of 77-353 K. Temperature-dependent PL decay curves of (d) Tb 3+ and (e) Eu 3+ and (f) temperature-dependent PL lifetimes for the sample with x = 0.001 in the 77-353 K range.